During a week in which man's inhumanity to man has been particularly lurid, a little affirmation, from whatever source, of our capacity for nobler behaviour would be welcome. It is to be found a thousand times over at the registration centre for today's Flora London Marathon. Steve O'Keefe, for one, has a story that is as worth telling as any deranged gunman's.

On 23 October 1990, O'Keefe was catapulted 60 feet when the van in which he was travelling as a passenger crashed near his home in Egham, Surrey. 'The van hit a kerb. It just happened,' he says, his speech still distorted as a result of the terrible brain damage he suffered. 'I landed on my head in the woods somewhere.'

At the hospital, doctors thought he would die that night. He did not, and a month later he came out of a coma. He could not remember a thing about the accident and can recall little about his thoughts on regaining consciousness. 'I thought it was a bad dream, a nightmare. Every single morning it was like, "I want to wake up in the morning, I really will wake up in the morning." I couldn't believe it was happening to me. I really couldn't. It was horrible, terrible.

'I couldn't talk, I couldn't feed myself, I couldn't wipe my own bum. There were so many things I couldn't do. The doctors said they didn't have a lot of hope. It was really bad. I was no good.'

This morning, O'Keefe, 35, who was in hospital for a year after the accident, will be on the starting line in Greenwich for his second marathon inside a week, having run the Boston last Monday. He will run three more marathons later in the year - in Stockholm, Berlin and Amsterdam - and says he has something planned for next year but does not want to say what it is just in case it does not come off.

Excited by what he has in mind, he cannot keep it a secret for long. He is doing two marathons in a week - the Boston took him four hours 56 minutes - to prepare for an ultra-distance race in 2008, although he has still to decide which one. He considered the 56-mile Comrades race in Durban, South Africa, the best-known of the ultra events, before deciding it would probably be too hot. 'I've got ginger hair, so I burn easily.'

Once the medical professionals had put O'Keefe on the long haul back from his near-vegetative state, the rest was a collaborative effort. He was an only child but says the support of his extended family and friends was crucial. 'When you have a head injury, it's very hard. You're left to yourself. But I've got a strong family and everyone about me has been absolutely brilliant.' He says it is easy to underestimate what is involved in recovering from brain damage, particularly if you watch too many soap operas. 'It's not like watching somebody on EastEnders or one of the other soaps who has had a head injury and all of a sudden in a few months they are better. It took me 10 years and I'm still getting better.

'It's just been a long process of rehabilitation: speech therapy, physiotherapy and my own occupational therapy, things like word processing and floristry just to get dexterity back in my fingers. I had no use of my fingers after the accident. I also did a lot of voluntary work for old people.'

After nine years of painstaking rehabilitation, O'Keefe had to cope with another bad experience when his mother, Kathy, was diagnosed with bowel cancer and it was this that gave him the idea to run marathons. He said he wanted to do something to make her feel proud of him and running marathons to raise money for cancer research 'seemed a cool thing to do'.

He has not missed a London Marathon since his first in 2003 - his fastest was 4hr 18min 2sec in 2005 - and he has also done New York and Boston, raising more than £56,000 for the Bobby Moore Fund for Cancer Research UK, a figure that is poised to go past £60,000 after today. His mother's cancer is now in remission.

Among the many who have been generous to him is the businessman Richard Caring, who owns Wentworth Golf Club, where O'Keefe works as a doorman and is a popular figure. So, too, have the Indian hotelier Surinder Arora, the club's vice chairman, and celebrities who play there, such as Michael Parkinson and the 2002 Open champion Ernie Els, whose fitness trainer Josh Saltzman has helped O'Keefe to regain strength.

O'Keefe, who was training to be a river-boat skipper before the accident, was a keen sportsman in his previous life: a better-than-average triple jumper, basketball player, distance runner and golfer who had a handicap of nine within a year of first swinging a club. All this was taken away in 1990 and only some of it has been regained; the running, of course, and he plays a little golf at Wentworth, although he is no longer the player he was. He would like to break 100, which he could once do easily.

He has taught himself to run again in defiance of medical opinion that rated his chances of even walking as slim. 'Someone once said to me, "Steve, you'll never run 26 miles, it's a long way." I said to him, "No, 26 miles is not a long way - two steps is a long way when you can't walk." That statement I think is the best statement. When you can't walk, 26 miles is nothing. That's why I want to run ultra marathons.'

O'Keefe still has a limp because of the ataxia, a loss of movement common to stroke sufferers, down his left side and describes himself as 'a bit lopsided'. He says, though, that, strange as it may seem, he now finds it easier to run than walk. 'It's like a pendulum - once it's going it just keeps on going. Once I'm up and started and get into the flow and movement I'm all right.'

The question of whether he is a driven person is superfluous, worth asking all the same to hear precisely what he sees as the purpose of this drive, beyond the obvious one of raising money for cancer research. The answer spills out, chopping between his desire to help others and himself. 'Oh yes, I'm completely driven. I know what I want, I know what I want to do, I know what I want to achieve.' What do you want to achieve? 'I want to run a long way. I've got a goal.' Which is? 'I hope I can inspire a lot of people who have had head injuries that there is a life.

'It's taken me nearly 20 years. It's hard. I want to keep pushing myself. I can get better, you know what I mean. I feel that my running is making me better, mentally and physically. I'm breathing better. I want to inspire other people.'

He pauses before saying one last thing. 'Please make me a good interview.'